Address problems with the 256 maxfd limit in pollset_create()

This was causing scaling failures when running a number of
system tests that drive high load.

I have changed the call to pass -1 as maxfd which should
mean the size of the pollset is limited only by the
user's ulimit and the global limit as described in the
pollset_create() documentation.

I have added some error messages giving failure details
in places where the code abort()s.